Уласцівасці літой нержавеючай сталі

1. Уводзіны

Cast stainless steels combine corrosion resistance, добрая механічная трываласць і ліцейнасць для складаных формаў.

They are used where corrosion, тэмпература, or sanitary requirements preclude ordinary carbon steels and where fabrication of complex geometry from wrought plate would be costly or impossible.

Performance depends on alloy family (аўстэнітны, дуплекс, ферытны, мартенситный, ападкава-загартоўвае), метад ліцця, heat treatment and quality control.

Proper specification and process control are essential to avoid embrittling phases and casting defects that can negate the metal’s intrinsic advantages.

2. Core Definition & Classification of Cast Stainless Steel

Core definition — what we mean by “cast stainless steel”

Адліваць з нержавеючай сталі refers to chromium-bearing iron alloys that are produced by pouring molten alloy into a mold and allowing it to solidify, then finishing and heat-treating as required.

The defining feature that makes them “stainless” is a sufficient chromium content (and often other alloying elements) to form and maintain a continuous, self-healing chromium oxide (Cr₂O₃) film that dramatically reduces general corrosion.

Castings are used where complex geometry, integral features (праходы, bossing, рэбра), or economic advantages of casting outweigh the benefits of wrought fabrication.

Family-by-family summary (стол)

Naming conventions & common cast grades (practical note)

• Cast stainless grades often use casting designations rather than wrought numbers (Напрыклад: CF8 ≈ 304, CF8M ≈ 316 equivalents in many specifications).
  These casting codes and alloy names vary by standard system (Астм, У, Ён, і г.д.).
• “CF” / “CA” / “CD” prefixes are typical in some standards to denote cast austenitic/ferritic/duplex groupings; manufacturers may also use proprietary names.
  Always specify both the chemical range і mechanical/heat-treatment requirement in procurement documents to avoid ambiguity.

3. Metallurgy and Microstructure

Alloy families and their defining features

• Аўстэніт (e.g., 304, 316, CF8/CF3 equivalents in cast): face-centered-cubic (FCC) iron matrix stabilized by nickel (or nitrogen).
  Excellent toughness and ductility, outstanding general corrosion resistance; susceptible to chloride pitting and stress-corrosion cracking (SCC) in some environments.
• Дуплекс (e.g., 2205-type cast equivalents): roughly equal ferrite (целацэнтрычны куб, БКК) + austenite phases.
  Высокая сіла, superior pitting/crevice resistance and better resistance to SCC than austenitics due to lower chromium-depleted zone formation; requires control of cooling to avoid brittle phases.
• Ферытныя: mostly BCC chromium-stabilized; better stress-corrosion performance in some environments, lower toughness at low temp compared with austenitics.
• Мартэнічны: heat-treatable, can be made very strong and hard, moderate corrosion resistance compared with austenitic and duplex; used for wear-resistant cast parts.
• Загартоўка ападкамі (Ph): alloys that can be age hardened (Ni-based or stainless PH grades), offering high strength with reasonable corrosion resistance.

Critical microstructural concerns

• Carbide precipitation (M₂₃C₆, M₆C) і сігма (а) фаза formation occur when castings are held too long in the 600–900 °C range (or cooled slowly through it).
  These brittle, chromium-rich phases deplete the matrix of chromium and reduce toughness and corrosion resistance.
• Intermetallics and inclusions (e.g., silicides, сульфіды) can act as crack initiators.
• Сегрэгацыя (chemical non-uniformity) is inherent to casting and must be minimized by melt and solidification control and sometimes homogenization heat treatments.

4. Physical properties of Cast Stainless Steel

Values above are representative engineering ranges. Always consult supplier data for specified grade, casting route and heat-treatment state.

5. Электрычны & Magnetic Properties of Cast Stainless Steel

• Электрычны супраціў: Austenitic cast stainless steels (Cf8, Cf3m) have high resistivity (700–750 nΩ·m at 25°C)—3× higher than cast carbon steel (200 нΩ·м).
  This makes them suitable for electrical insulation applications (e.g., карпусы трансфарматараў).
• Магнетызм: Аўстэнітныя маркі (Cf8, Cf3m) быць немагнітны (relative permeability μ ≤1.005) due to their FCC structure—critical for medical devices (e.g., MRI-compatible components) or electronic enclosures.
  Ферытныя (CB30) і мартенситные (CA15) маркі ферамагн, абмежаванне іх выкарыстання ў магнітных адчувальных асяроддзях.

6. Casting processes and how they affect properties

Common casting routes for stainless:

• Пясчанае ліццё (Зялёны пясок, resin sand): flexible for large or complex parts.
  Coarser microstructure and higher risk of porosity unless controlled. Suitable for many pump bodies and large valves.
• Інвестыцыя (Страчаны WAX) ліццё: excellent surface finish and dimensional accuracy; often used for smaller, complex parts requiring tight tolerances.
• Цэнтрабежнае ліццё: produces sound, fine-grained cylindrical parts (трубы, рукавы) with directional solidification that minimizes internal defects.
• Shell and vacuum casting: improved cleanliness and reduced gas entrapment for critical applications.

Process influences:

• Cooling rate affects dendrite spacing; больш хуткае астуджэнне (інвестыцыя, цэнтрабежны) → finer microstructure → generally better mechanical properties.
• Melt cleanliness and pouring practice determine inclusion and bifilm levels that directly influence fatigue and leak tightness.
• Directional solidification and risering design minimize shrinkage cavities.

7. Mechanical properties of Cast Stainless Steel

Strength and ductility

• Austenitic castings: good ductility and toughness; UTS typically in mid hundreds of MPa; ductility high (elongation often 20–40% in cast 316L when free of defects).
• Duplex castings: higher yield and UTS due to ferrite + аустениты; typical UTS ~600–900 MPa with yield often >350 МПА.
• Martensitic/PH castings: can reach very high UTS and hardness but with reduced ductility.

Стомленасць

• Fatigue life is very sensitive to casting defects: сітаватасць, уключэнні, surface roughness and shrinkage are common crack starters.
  For rotating or cyclic loads, low-porosity processes, Стрэл Пінінг, Бядро (гарачае ізастатычнае прэсаванне), and surface machining are commonly used to improve fatigue performance.

Creep and elevated temperature

• Some stainless grades (especially high-alloy and duplex) retain strength at elevated temperatures; however long-term creep performance needs to be matched to alloy and expected life.
  Carbide/σ-phase precipitation under thermal exposure can severely reduce creep and toughness.

8. Тэрмічная апрацоўка, microstructure control and phase stability

Адпал раствора (тыповы)

• Намер: dissolve undesirable precipitates and restore a uniform austenitic/ferritic matrix; recover corrosion resistance by returning chromium to solid solution.
• Typical regime: heat to the appropriate solution temperature (often 1,040–1,100 °C for many austenitics), hold to homogenize, затым хуткае гашэнне to retain the solved-in elements. Exact temperature/time depends on grade and section thickness.
• Caveat: crucible and section size limit achievable quench rates; heavy sections may require special procedures.

Aging and precipitation

• Дуплекс і мартенситный grades may be aged for property control; aging/time–temperature windows must avoid sigma and other deleterious phases.
• Overaging or inappropriate thermal histories produce carbides and sigma that embrittle and reduce corrosion resistance.

Avoiding sigma phase and chromium depletion

• Control cooling through the vulnerable temperature range, avoid prolonged hold between ~600–900 °C, and use post-weld or solution annealing where needed.
  Material selection and heat treatment design are the main defenses.

9. Corrosion Resistance — Core Advantage of Cast Stainless Steel

Corrosion resistance is the primary reason engineers choose cast stainless steel.

Unlike many structural metals that rely on bulky coatings or sacrificial protection, stainless steels gain durable environmental resistance from their chemistry and surface reactivity.

How stainless steels resist corrosion — the passive film concept

• Passive protection: Chromium in the alloy reacts with oxygen to form a thin, continuous chromium-oxide layer (Cr₂O₃).
  This film is only nanometres thick but is highly effective: it reduces ionic transport, blocks anodic dissolution, and—crucially—is self-healing when damaged provided oxygen is available.
• Alloy synergy: Нік, molybdenum and nitrogen stabilize the matrix and improve the passive film’s resistance to local breakdown (асабліва ў хларыдных асяроддзях).
  The passive film’s stability is therefore an outcome of chemistry, Умова паверхні, and local environment.

Forms of corrosion that matter for cast stainless steels

Understanding likely failure modes focuses material selection and design:

• Агульны (уніформа) карозія: Rare for properly alloyed stainless in most industrial atmospheres — the passive film keeps uniform loss very low.
• Точкавая карозія: Лакалізаваны, often small and deep pits initiated when the passive film breaks down locally (chlorides are the classic initiator). Pitting can be critical because small defects penetrate quickly.
• Crevice corrosion: Occurs inside shielded gaps where oxygen becomes depleted; the oxygen gradient encourages local acidification and chloride concentration, undermining passivity inside the crevice.
• Каразійнае парэпанне пад напругай (SCC): A brittle cracking mechanism that requires a susceptible alloy (commonly austenitic stainless in chloride environments), напружанне расцяжэння, and a specific environment (цёплы, chloride-bearing). SCC can appear suddenly and catastrophically.
• Microbially influenced corrosion (MIC): Biofilms and microbial metabolism (e.g., sulfate-reducing bacteria) can produce localized chemistries that attack stainless castings, particularly in stagnant or low-flow crevices.
• Erosion-corrosion: Combination of mechanical wear and chemical attack, often where high velocity or impingement strips protective film and exposes fresh metal.

The role of alloying — what to specify and why

Certain elements strongly influence localized corrosion resistance:

• Хром (Кр): Foundation of passivity; minimum content defines “stainless” behavior.
• Molybdenum (Мо): Very effective at increasing pitting and crevice resistance — essential for seawater and chloride service.
• Азот (N): Strengthens austenite and greatly improves pitting resistance (efficient small additions).
• Нік (У): Stabilizes austenite and supports toughness and ductility.
• Copper, вальффральф, Nb/Ti: Used in specialized alloys for niche environments.

A useful comparative index is the Pitting Resistance Equivalent Number (Дрэва):

PREN=%Cr+3.3×%Mo+16×%N

Typical PREN (rounded, прадстаўнік):

• 304 / Cf8 ≈ ~19 (low pitting resistance)
• 316 / Cf8m ≈ ~24 (умераны)
• Дуплекс 2205 / CD3MN ≈ ~35 (высокая)
• Супераустенитный (e.g., high-Mo / 254SMO equivalents) ≈ ~40–45 (вельмі высокая)

Практычнае правіла: higher PREN → greater resistance to chloride-induced pitting/crevice corrosion. Pick PREN proportional to exposure severity.

Environmental drivers — what makes stainless fail

• Хларыды (sea spray, антигололедные солі, chloride-bearing process streams) are the dominant external threat — they promote pitting, crevice corrosion and SCC.
• Тэмпература: Elevated temperatures accelerate chemical attack and SCC susceptibility; the combination of chloride + elevated temperature is particularly aggressive.
• Stagnation & шчыліны: Low oxygen and confined spaces concentrate aggressive ions and destroy local passivity.
• Mechanical stress: Tensile stresses (residual or applied) are necessary for SCC. Design and stress relief reduce risk.
• Microbial life: Biofilms modify local chemistry; MIC is particularly relevant in wet, poorly flushed systems.

Задума & specification strategies to maximise corrosion resistance

1. Right-grade selection: Match PREN/chemistry to exposure — e.g., 316 for moderate chlorides, дуплекс / high-Mo grades for seawater or chloride-rich process streams.
2. Control thermal history: Require solution anneal + quench where indicated; specify maximum cooling times in the σ-formation window for duplex grades.
3. Якасць паверхні: Specify surface finish, electropolishing or mechanical polishing for sanitary or high-pitting-risk components; smoother surfaces reduce pit initiation.
4. Detailing to avoid crevices: Design to eliminate tight crevices, provide drainage and allow inspection access. Use gasketing, sealants and proper fastener selection where joints are unavoidable.
5. Welding practice: Use matched/over-alloyed filler metals, кантроль паступлення цяпла, and specify PWHT or passivation as needed. Protect welds from post-weld sensitization.
6. Dielectric isolation: Electrically isolate stainless parts from dissimilar metals to prevent galvanic acceleration of corrosion.
7. Пакрыцці & накладкі: When environment exceeds even high-alloy capability, use polymer/ceramic linings or claddings as first line (or as backup) — but do not rely on coatings alone for critical containment without inspection provisions.
8. Avoid tensile stress in SCC-sensitive environments: Reduce design stresses, apply compressive surface treatments (Стрэл Пінінг), and control operating loads.

10. Выдумка, Далучэнне, and Repair

Вінжаванне

• Cast stainless steels are generally зварваецца, but attention is needed:

• • Match filler metal to base alloy or select a more corrosion-resistant filler to avoid galvanic effects.
  • Preheat and interpass control for some martensitic grades to manage hardness and cracking risk.
  • Послесварочный адпал раствора is often required for austenitic and duplex fillers to restore corrosion resistance and reduce residual stresses.
  • Avoid slow cooling that can produce σ-phase.

Апрацоўванне

• Machinability varies: austenitic stainless steels work-harden and require sharp tooling and appropriate speeds; duplex grades cut better in some cases due to higher strength. Use appropriate coolant and cutting parameters.

Аздабленне паверхні

• Pickling and passivation restore chromium oxide and remove free iron contaminants.
  Electrochemical polish or mechanical finishing improves cleanliness, reduces crevice sites and boosts corrosion resistance.

11. Эканамічны, lifecycle and sustainability considerations

• Каштаваць: cast stainless steel raw material cost is higher than carbon steel and aluminum, and casting requires higher melting temperatures and refractory costs.
  Аднак, the life extension and reduced maintenance in corrosive environments can justify the premium.
• Жыццёвы цыкл: long service life in corrosive environments, lower replacement frequency and recyclability (stainless scrap value is high) improve lifecycle economics.
• Устойлівасць: stainless alloys contain strategically important elements (Кр, У, Мо); responsible sourcing and recycling are essential.
  Energy for initial production is high, but recycling stainless significantly reduces embodied energy.

12. Параўнальны аналіз: Літая нержавеючая сталь супраць. Competitors

13. Высновы

Cast stainless steel occupies a unique and strategically important position among structural and corrosion-resistant casting materials.

A single property does not define its value, but by the synergistic combination of corrosion resistance, Механічная сіла, тэрмаўстойлівасць, versatility in alloy design, and compatibility with complex casting geometries.

When evaluated across performance, надзейнасць, and lifecycle metrics, cast stainless steel consistently proves to be a high-performance solution for demanding industrial environments.

Увесь, cast stainless steel stands out as a high-integrity, рознабаковы, and reliable material choice for industries requiring corrosion resistance, mechanical durability, and precision castability.

 

FAQ

Is cast stainless as corrosion-resistant as wrought stainless?

Гэта можа быць, but only if the casting chemistry, microstructure and heat treatment meet the same standards.

Castings have more opportunity for segregation and precipitates; solution anneal and rapid quench are often required to restore full corrosion resistance.

How do I avoid sigma phase in castings?

Avoid long holds between ~600–900 °C; design heat treatments to solution anneal and quench, and select alloys less prone to sigma (e.g., balanced duplex chemistries) for hostile thermal histories.

Which cast stainless should I pick for seawater service?

High-PREN duplex alloys or specific super-austenitics (вышэй Пн, N) are typically preferred. 316/316L may be inadequate in splash zones or where oxygenated seawater flows at high velocity.

Are cast stainless components weldable on site?

Так, but welding may locally alter metallurgical balance. Post-weld heat treatment or passivation may be needed to restore corrosion resistance near welds.

What casting method gives the best integrity for critical parts?

Цэнтрабежнае ліццё (for cylindrical parts), investment/precision casting (for small complex parts) and vacuum or controlled-atmosphere mold casting combined with HIP provide the highest integrity and lowest porosity.

Is cast stainless steel suitable for high-temperature applications?

Аўстэнітныя маркі (Cf8, Cf3m) are usable up to 870°C; Дуплексныя адзнакі (2205) up to 315°C.

Для тэмпературы >870° С, use heat-resistant cast stainless steels (e.g., HK40, з 25% Кр, 20% У) або нікелевых сплаваў.